The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed the Global System for Mobile communication (GSM). 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a user equipment (UE) is wirelessly connected to a radio base station (RBS) commonly referred to as an eNodeB (eNB). The eNBs are directly connected to a core network (CN).
In a wireless communication system using Orthogonal Frequency Division Multiplexing (OFDM) technology, the entire channel is divided into many narrow sub-channels, which are transmitted in parallel. This technique thus transforms a frequency selective wide-band channel into a group of non-selective narrowband channels, making it robust against large delay spread by preserving the orthogonality in the frequency domain. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions without complex equalization filters in the receiver. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate inter-symbol interference.
In an LTE system the OFDM technology is adopted as a mean to achieve high transmission capability and robustness to multi-path delay. Orthogonal Frequency Division Multiple Access (OFDMA) is used in the downlink, and Single-carrier Frequency Division Multiple Access (SC-FDMA) is used in the uplink. OFDMA is a multi-user version of OFDM, where multiple access is achieved by assigning subsets of sub-carriers to individual users. SC-FDMA is a linearly pre-coded OFDM scheme. The distinguishing feature of SC-FDMA is that it leads to a single-carrier transmit signal, in contrast to OFDMA which is a multi-carrier transmission scheme. Furthermore, SC-FDMA has a lower peak-to-average power ratio which entails improved transmitter power efficiency for the battery-operated UE.
In LTE downlink (DL), the physical layer is thus based on OFDMA. The information to be transmitted is coded e.g. by a turbo coding, interleaved, scrambled, and modulated to symbols. Some examples of modulation schemes are the Phase Shift Keying (PSK) modulations such as Quaternary or Quadrature PSK (QPSK), and the combinations of PSK and Amplitude Shift Keying (ASK) modulations such as 16 Quadrature Amplitude Modulation (QAM) and 64QAM. The symbols are fed to an Inverse Fast Fourier Transform (IFFT), where these symbols are mapped to a specified frequency interval specified as a number of sub-carriers. A resource block consists of 12 sub-carriers and is the smallest amount that a UE can be allocated. The IFFT is used to transform the symbols to be transmitted from a frequency domain representation to a time domain representation.
In LTE uplink (UL), the physical layer is based on SC-FDMA, which is also referred to as pre-coded OFDM. This means that the physical channels are built of SC-FDMA symbols. The modulated symbols are transformed to the frequency domain by a Discrete Fourier Transform (DFT) of the same size as the number of modulated symbols of each SC-FDMA symbol. This is then fed to a larger IFFT with a size which depends on the bandwidth of the radio communication link.
A radio communication between a UE and an RBS will be affected by multi-path propagation, fading, frequency errors, round trip times etc. This communication channel is often referred to as an air interface, and causes bit and block errors on information transmitted. A receiver is designed in order to reduce bit error and block error rates, and comprises e.g. FFTs, channel estimators, an equalizer and an antenna combining unit. In brief the LTE UL Layer 1 receiver chain consists of the following steps:
1. FFT to extract users, done per antenna
2. Channel estimation, done per antenna and user
3. Antenna Combination, combining signals from different antennas
4. Equalization using channel estimates
5. IFFT
6. Soft De-mapping, de-mapping symbols to soft values
7. Decoding
In LTE the equalization is based on the channel estimation and the purpose of the equalization is to compensate for channel distortion. Different methods for equalization may be used depending on the characteristics of the channel distortion. The most common method is linear Minimum Mean Square Error (MMSE) equalization. However, an MMSE equalizer does not provide good performance when the channel distortion includes non-negligible inter-symbol-interference (ISI). Inter-symbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols cause disturbances, thus making the communication less reliable.
To combat the impact of ISI in SC-FDMA transmitted signals and Enhanced General Packet Radio Service (EGPRS) transmitted signals respectively, the articles Berardinelli, G., Priyanto, B. E., Sorensen, T. B., and Mogensen, P., “Improving SC-FDMA Performance by Turbo Equalization in UTRA LTE Uplink” Vehicular Technology Conference, 2008. VTC Spring 2008 and C. Laot, R. Le Bidan and D. Leroux, “Low-complexity MMSE turbo equalization: A possible solution for EDGE,” IEEE transactions on wireless communications, vol. 4, No. 3, pp. 965-974, May 2005. IEEE, pp. 2557-2561, May 2008 propose using turbo equalization methods. According to these a turbo equalizer that involves a combination of decoding and equalization is used to deal with the ISI in the received radio signals. The idea of such a turbo equalizer is to make use of soft values that are output from the decoder, to improve the result of the equalizer so that ISI is significantly reduced in the signal to be decoded.
However, this algorithm involves iteratively re-modulating the soft bits or soft values of coded bits into symbols and feeding them back into the receiver chain from equalizer to decoder, which means high computational complexity and requires a lot of computational power of the Digital Signal Processor (DSP). Several iterations are needed in order to get acceptable performance gain in terms of increased throughput. There is however no guarantee that the turbo equalizer, or the turbo equalization mode of an equalizer capable of operation in more than one equalization mode, provides an acceptable gain if employed. Utilization of the turbo equalizer may therefore be prevented, as disclosed in U.S. Pat. No. 7,010,064 B2 to Penther, where the receiver switches between a branch for turbo equalization and a branch for equalization and turbo decoding depending on delay spread of the transmission channel. A switch in the receiver is controlled by an estimator which estimates the delay spread and compares it with a predetermined threshold. If the delay spread is above the threshold turbo equalization is selected. If the delay spread is below the threshold equalization by a soft-equalizer followed by decoding by a turbo decoder is selected. Thus, in U.S. Pat. No. 7,010,064 B2 channel delay spread is used as a criterion to determine if it is necessary to switch on or employ the turbo equalizer. Nevertheless, this measure does not solve the problem as the gain in throughput may vary even with the same channel delay spread. Additionally, in a realistic situation when only a limited number of iterations can be afforded, it is likely that the turbo equalizer could not guarantee error-free decoding or an acceptable gain in terms of throughput even if it is theoretically beneficial to switch it on.